Stent having variable properties and method of its use

ABSTRACT

A stent and method of its use, the stent in its expanded configuration, exhibiting varying outward radial force along its length. In use, the expanded stent is of a tapered configuration which provides greater force in vessel regions requiring greater force and less force in regions requiring less. In particular the stent is useful in the ostium regions and at areas of bifurcation in vessels. Varying force over the length of the stent is achieved by varying the number of elements, the density of elements, the thickness of the elements making up the stent body, and maintaining a substantially metal to artery ratio in the expanded stent over its length.

CROSS REFERENCE TO CO-PENDING APPLICATION

This application is a Divisonal of U.S. application Ser. No. 09/034,249,filed Mar. 4, 1998, now U.S. Pat. No. 5,938,697 which is incorporatedherein in its entirety by reference.

FIELD OF THE INVENTION

The invention relates generally to medical devices and their use. Morespecifically, the invention relates to stents for holding vessels suchas arteries open to flow, particularly in the regions of bifurcations.

BACKGROUND OF THE INVENTION

Stents are radially expandable endoprostheses which are typicallyintravascular implants capable of being implanted transluminally andenlarged radially after being introduced percutaneously. They have alsobeen implanted in urinary tracts and bile ducts. They are used toreinforce body vessels and to prevent restenosis following angioplastyin the vascular system. They may be self-expanding or expanded by aninternal radial force, such as when mounted on a balloon.

Stents are generally tubular devices for insertion into tubular vesselregions. Balloon expandable stents require mounting over a balloon,positioning, and inflation of the balloon to expand the stent radiallyoutward. Self-expanding stents expand into place when unconstrained,without requiring assistance from a balloon. A self-expanding stent isbiased so as to expand upon release from the delivery catheter.

A vessel having a stenosis may be viewed as an inwardly protrudingarcuate addition of hardened material to a cylindrical vessel wall,where the stenosed region presents a somewhat rigid body attached along,and to, the elastic wall. The stenosis presents resistance to anyexpansion of the vessel in the region bridged by the stenosis. Stenosesvary in composition, for example, in the degree of calcification, andtherefore vary in properties as well.

The arcuate geometry of any stenoses present a variation in resistancealong the vessel axis to stent outward radial force. Specifically,stenosed vessel resistance is often greatest toward the middle,lessening toward the ends, with a rapid decrease at the start of healthyvessel tissue.

In some instances, as in regions of bifurcation, stenoses are believedto be flow related phenomena, see Chapter 21 of the "Handbook ofBioengineering" (Richard Shaloh & Shu Chin, McGraw-Hill Book Company,1987) which discusses atherosclerosis at vascular bifurcations.

The left and right common carotid arteries are typical of such vascularbifurcations. These arteries are the principal arteries of the head andneck. Both of the common carotid arteries are quite similar and divideat a carotid bifurcation or bulb into an external carotid artery and aninternal carotid artery. In the region of the carotid bulb and theostium of the internal carotid artery, stenoses present a particularproblem for carotid stenting due to the large tapering of the vesselinterior from the common carotid artery (both the left and the right) tothe internal carotid artery. The region of the carotid bifurcation orbulb happens to be where stenoses most often occur, particularly in theregion of the ostium to the internal carotid artery in both of thecarotid arteries. Self-expanding stents are generally preferred forcarotid stenting due to the anatomical location being subject toexternal compression.

A conventional self-expanding stent optimally has a length greater thanthe length of the stenosed region to be kept open. Current stentspresent a substantially uniform outward radial force and a uniformresistance to compression along their length. Currently, stents do notvary these forces to match vessel geometries or resistances. A constantforce stent, i.e., prior art stents, with sufficient force to maintainan open channel within a stenosed vessel and to resist compression, hasgreater force than necessary in the healthy vessel portion distal to thestenosis. The stent end may thus flare outward, protruding into, andpossibly irritating non-stenosed tissue.

Stenoses can occur in vessel regions having asymmetric geometry lying oneither side of the stenosis. One example of this is the ostium of aninternal carotid artery, having a wide opening converging into anarrower artery. A conventional stent placed in the region of the ostiumwould provide substantially uniform outward radial force over anon-uniform vessel diameter, that is, the force provided would begreater in a small diameter than in a larger diameter. If this force isproperly matched for the smaller vessel region, it is likely less thanoptimal for the larger region. Conversely, if this force is properlymatched for the larger vessel region, it is likely more than optimal forthe smaller vessel region.

What would be desirable, and has not heretofore been provided, is atapered stent capable of providing sufficient force to keep a vesselopen within a rebounding stenosis, while providing only necessary forceagainst healthy, non-stenosed vessel regions. What else has not beenprovided is a tapered stent providing necessary, but only necessaryforce (outward force and compression resistance) along a stenosis in avessel region having non-uniform vessel diameter on either side of thestenosis. This is provided by the tapered stents of this invention whichexhibit differing radial force, cell size, geometry, flexibility andwhich provide substantially more constant metal to artery ratio (M/A)over their length. M/A is the ratio of the metal surface area of a stentto the surface area of the vessel or the like that the stent iscovering.

SUMMARY OF THE INVENTION

The present invention, in a preferred embodiment, includes aself-expanding stent of shape-memory metal having a tubular orcylindrical shaped structure in the unexpanded condition and a taperedtubular or cylindrical structure in the expanded or memorized condition,and in which the radial force varies longitudinally along the length ofthe stent. Also, its resistance to compression varies with length.Additionally, the cell design making up the stent is closed where forceand good plaque coverage and support is required and open whereflexibility is required. Additionally, the metal to artery ratio issubstantially more constant over the length of the stent when it isexpanded. One such stent is constructed of Nickel-Titanium alloy(nitinol). Other shape memory metals may be used. In one embodiment, thestent is constructed and arranged so that the outward radial force isgreater in the center and lesser at both ends. In another embodiment,the stent is constructed and arranged so that the outward radial forceis greater at one end and less at the opposite end. Such stents aresuitable for placement in stenosed and narrowing vessel regions such asthe carotid bifurcation and the ostial area associated therewith.

The stents of the invention may achieve a variation in radial forcealong their length by including in the stent structural elements whichintersect at connections having more metal in regions requiring moreradial force and less metal in regions requiring less radial force. Theamount of intersection metal or strut member metal can be varied byvarying the size of the intersection area or the size of the struts.Greater or fewer connectors actually are used to vary the flexibilityalong the length of the stent more than increasing radial force. In apreferred embodiment, the stent structure is formed by laser cutting aNitinol tube, leaving a greater strut width and shorter length inregions requiring greater outward radial force and compressionresistance.

The struts of the invention are also characterized by the fact that theyare constructed and arranged to present a substantially more constantmetal to artery ratio over their length in the expanded condition, i.e.,expanded to a tapered shape.

The stent structure in a preferred embodiment includes a series ofserpentine annular segments which are aligned to provide a tubularstructure. The segments are interconnected longitudinally. A desiredradial force can be varied by varying the stent strut dimensions in thisand other embodiments. In one embodiment, stent regions requiringgreater radial force have wider and shorter struts than regionsrequiring less force. The number of connectors between segments can alsobe varied for this purpose. It is also obtained by varying strut lengthand spacing and overall size. Another control is cell design per se.Closed cells provide greater plaque coverage and support than opencells. Closed cells are generally connected to cells in adjoiningsegments of the stent whereas open cells are not so connected. Thesefactors also provide control over properties such as flexibility andconformability. Cell geometry, i.e., closed and open, is used to providegood plaque support in the region of the stenoses (closed) and lesssupport (open) and more flexibility to either side of the stenoses.Also, closed cell structure may be used to bridge the origin of theexternal carotid artery or any other vessel side branch opening.

Generally speaking it is desirable to provide a stent of this inventionwith the aforementioned radial force which is variable over stent lengthin a predetermined arrangement; cell design which is closed in the areawhere the stent contacts plaque of a stenoses and more open where thestent contacts healthy vessel tissue; flexibility and conformabilitywhich is arranged to vary in a predetermined arrangement over the lengthof the stent, in both unexpanded and expanded condition.

Stents made in accordance with the present invention can provide anoutward radial force more closely matching the local force requirementsin a tapered vessel. In particular, the stents provide greater forceonly where required at a stenosis, without providing too much force inthe region of healthy tissue. The stents provide an expanded geometrymore closely tailored to the requirements of a tapering vessel region.They are preferably stiff and strong at the proximal large diameter endor middle and weak and more flexible at the distal smaller diameter endto provide strain relief and prevent kinking of the vessel distal to thestent. The proximal end may also be flexible.

A stent of the invention with variable properties along its length alsoapplies to balloon expandable stents that can be used acrossbifurcations with large diameter change by dilating with a smallerballoon distally and a larger balloon proximally.

This invention is also concerned with a method for treating stenoses invessel bifurcation regions involving the use of a stent of the typedescribed above.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic showing of a scenario 1 for carotid stenting;

FIGS. 2a and 2b are plots of force versus length of improved stents forplacement in FIGS. 1 and 7 respectively, i.e., an ostial stent and abifurcation stent;

FIG. 3 is a schematic profile view of an expanded, tapered stent for usein the scenario 1 of FIG. 1;

FIG. 4 is a flat plan view in detail of an unexpanded stent of the typeshown schematically in FIG. 3, including exemplary dimensions;

FIGS. 4a, 4b, 4c and 4d are detail showings of portions of FIG. 4;

FIG. 5 is an end view of the stent of FIG. 4;

FIG. 6 is a view showing the stent of FIG. 4 in the expanded condition;

FIG. 7 is a schematic of a scenario 2 for carotid stenting;

FIG. 8 is a schematic profile view of an expanded, tapered stent for usein the scenario 2 of FIG. 7;

FIG. 9 is a flat plan view in detail of an unexpanded stent of the typeshown schematically in FIG. 8, including exemplary dimension, and

FIG. 10 is an end view of the stent of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a narrowing vessel 52, such as the internal carotidartery, having a wide region 56, a narrowed region 58, and a stenosis(not shown) somewhere in between, i.e., in the cross-hatched region. Thenarrowing vessel of FIG. 1 illustrates the geometry as found in anostium at the bifurcation of the left common carotid 57, where bloodflows from the left common carotid artery 57 into the left internalcarotid artery 59. The bifurcation also opens into the left externalcarotid artery 60. An ordinary stent with sufficient force to hold openthe wide region 56 would have greater force than necessary to hold openthe narrowed region 58.

FIG. 2a illustrates a plot 66a of outward radial force F along atapered, expanded stent length L for a stent embodying the presentinvention. The stent has a greater force in end region 68a than at theopposite end region 70a. A tapered stent having the force curve of FIG.2a is suitable for bridging a stenosis as illustrated in FIG. 1, havingsufficient force to hold open the wide region 56 of a vessel and lessforce in the narrow healthy tissue region 58 of the vessel, where lessis required.

FIG. 3 illustrates in schematic fashion a preferred nitinol stentembodiment of the invention producing a force distribution asillustrated in FIG. 2. Self-expanding stent 80 includes a conformabledistal end 82 for contacting healthy vessel tissue, and a stiffer,closed-cell proximal region 88 for providing increased plaque support.It has upon expansion a tapered diameter as shown. For example, a 0.236inch distal diameter and a 0.354 inch proximal diameter might betypical. These dimensions can be varied. Stent 80 is positioned on thedistal end of a delivery catheter, covered with a removable sheath,advanced to a stenosis to be crossed, and exposed for self-expansion byremoval of the sheath. Stent 80 expands radially to its memorizedtapered shape pushing against the stenosis and vessel wall.

FIG. 4 illustrates in more detail the nitinol unexpanded stentembodiment of FIG. 3 in flat plan view as a stent 100, having a middleregion 104 and end regions 106 and 108. Stent 100 has a tubular shape,shown in FIG. 5, formed of several serpentine segments 105, 107, 109,111 and 113, having a zig-zag pattern, each segment radially encirclinga portion of stent 100. Referring again to FIG. 4, segments 113 arelongitudinally interconnected by connectors 110, whereas the serpentinesegments 105, 107, 109 and 111 are all interconnected as shown in FIGS.4a and 4b by direct connections 112. A preferred material forconstructing stent 100 is Nitinol. In this embodiment, the stent isformed by laser cutting a continuous-walled nitinol tube of diameter0.081 inches having a wall thickness of 0.006 inches, leaving only thestent structure as shown. Typical dimensions of various elements of thestent are shown in the Figure by way of example.

Referring now to FIG. 6, the stent of FIG. 4 is shown expanded andtapered. Since nitinol is a shape memory metal it can be formed into theshape and size shown in FIG. 4, placed over a tapered tool and expandedto a desired enlarged shape and size, such as the 0.236 inch distaldiameter and 0.354 inch proximal diameter previously mentioned, heatedto a high temperature such as 500° C. to give it the memorized size andshape on the tool. The stent is then removed from the tool and can becompressed for mounting on the delivery catheter.

By starting with a stent of nitinol having the dimensions set forth inFIG. 4, the expanded condition provides a stent having the desirableproperties described hereinbefore with reference to FIG. 3. Alldimensions in the Figure are in inches. Of course, this is but oneexample of a stent according to the invention.

FIG. 7, similarly to FIG. 1, illustrates a narrowing vessel 52 having awide region 56, a narrowed region 58, a branching vessel 55 and astenosis (not shown) somewhere in between regions 56 and 58, i.e., thecross hatched region. Again, narrowing vessel of FIG. 7 illustrates thegeometry as found at the bifurcation of the left common carotid artery57, where blood flows from the left common carotid artery 57 into theleft internal carotid artery 59.

FIG. 2b illustrates a plot 66b of outward radial force F along atapered, expandable stent length L for a stent embodying the presentinvention. The stent has a greater force in its middle region 67b thanat its end regions 68b and 70b. A tapered stent having the force curveof FIG. 2b is suitable for bridging a stenosis as illustrated in FIG. 7,having sufficient force to hold open the wide region at the ostium ofinternal carotid 59 and less force in healthy tissue at wide end 56 andnarrow end 58.

A stent for use in this cross hatched region will have properties suchas those to be described with reference to FIGS. 8 and 9, which will bedifferent from the stent previously described with reference to FIGS.1-6.

Referring now to the FIG. 8 schematic, stent 80 includes a middle region84 and end regions 86 and 87. The amount of radial force exerted perunit length of stent is greater in regions having shorter and widerstruts. As schematically illustrated in FIG. 8, stent 80 has shorter andwider struts in center region 84 than in end regions 86 and 87. Thus,stent 80 has a greater outward radial force and compression resistancein center region 84 than in end regions 86 and 87 making it particularlyuseful for stenting in the cross-hatched region of FIG. 7.

FIG. 9 illustrates in more detail the nitinol unexpanded stentembodiment of FIG. 8 in flat plan view as a stent 100 having a middleregion 104 and end regions 106 and 108. Stent 100 has a tubular shape,shown in FIG. 10, formed of several serpentine segments 105, 107, 109,111 and 113, having a zig-zag pattern, each segment radially encirclinga portion of stent 100. Segments 111 and 113 are respectivelylongitudinally interconnected by several connectors 110 whereasserpentine segments 105, 107 and 109 are all interconnected as shown indetail in FIGS. 9a and 9b by direct connections 112. This embodiment isalso formed by laser cutting a continuous-walled nitinol tube ofdiameter 0.081 inches having a wall thickness of 0.006 inches, leavingonly the stent structure as shown. Typical dimensions of variouselements of the stent are shown in FIG. 9 by way of example.

Similarly to the stent embodiment of FIG. 4 as expanded to a taperedshape shown in FIG. 6, the stent of FIG. 9 can be provided with atapered memorized shape in the expanded condition. The stent willexhibit all of the desirable proportions heretofore described,particularly as discussed with reference to FIG. 2b. All dimensions inFIG. 9 are in inches.

The present invention provides a stent which when expanded to itstapered configuration, provides a radial force varied along stent lengthfor use in tapered anatomies. The stent has been described, in use, asbridging stenosed vessel regions for illustrative purposes. Another usein maintaining open channels through otherwise restricted body conduits.Stents used for other purposes are explicitly within the scope of theinvention.

It should be noted that although self-expanding stents have been shownherein to illustrate the present invention, so called balloon expandablestents can also include the variable radial force feature as describedherein. In the case of balloon expandable stents, however, these forcesin general will be less than are necessary to expand the stent and thusthe balloon will be used as known to those skilled in the art tocomplete the expansion of the stent. To obtain the tapered shape, twoballoons of different diameter may be used to expand each end of thestent. These balloon expandable stents may be advantageously deployed inareas of a vessel such as at an ostium where a stent having more rigidor heavy members is desirable in the region of the stenosis, andenhanced flexibility in the distal portion of the stent is desired. Forexample, a balloon expandable stent can be made of stainless steel tothe design and dimensions shown in either FIG. 4 or FIG. 9. It should beunderstood therefore, that balloon expandable stents are also within thescope of the present invention.

In use, a stent of the self-expanding type, in unexpanded form, isplaced on a delivery catheter and covered with a retractable sheath. Thecatheter is introduced into a vessel and advanced to a region ofbifurcation (ostium or bifurcation placement). The sheath is retracted,typically by pulling it in the proximal direction, to expose the stent.The stent then self-expands to contact the vessel wall and stenosis. Inthe case of a self-expanding stent such as the nitinol type describedherein, the stent expands to the tapered configuration upon beingexposed and exhibits the desired proportion described hereinbefore. Asheath is typically used for constraining a self-expanding stent. Aballoon expandable stent is typically crimped on to the balloon and notcovered by a sheath. In the case of a non-self-expanding stent, aballoon or other radial force means is inflated within the stent toexpand it. In the case of the stents described herein, two balloons maybe used sequentially to accomplish this. For example, a small balloonmay be used to expand the stent at the small diameter end of the taperedconfiguration. Then, a larger balloon may be used to expand the stentsat the large end of the tapered configuration. The catheter(s) arewithdrawn, leaving the stent implanted in the vessel. The method isadaptable depending on whether an ostial version or a bifurcationversion of the stent is being implanted.

Numerous characteristics and advantages of the invention covered by thisapplication have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many aspects, onlyillustrative. Changes may be made in details, particularly in matters ofshape, size, and arrangement of parts and in materials without exceedingthe scope of the invention. The invention's scope is, of course, definedin the language in which the appended claims are expressed.

What is claimed is as follows:
 1. A method of stenting a bifurcationregion or ostial region in a blood vessel, comprising the stepsof:providing a stent having an unexpanded configuration and an expandedconfiguration, the latter having a length and a varying diameter alongsaid length comprising a tapered tubular shaped metallic structure, saidstructure having a radially outward biased force and compressionresistance, said force varying along said length in a predeterminedmanner, said tapered tubular structure having a first end region oflargest diameter, a middle region of smaller diameter, and a second endregion of smallest diameter, wherein said outward biased force is weakerin said first end region, stronger in said middle region than in saidfirst region, and weaker in said second end region than in said middleregion; introducing the stent into a catheter; inserting the catheterand stent into an implantation region at bifurcation or ostium of avessel; expanding the stent to an enlarged diameter of a taperedconfiguration in support of the vessel wherein the aforementionedproperties are exhibited, and removing the catheter leaving the stentimplanted in the vessel.
 2. The method of claim 1 wherein the stent isimplanted in the bifurcation region of the common carotid artery,extending across the opening into the external carotid artery and intothe internal carotid artery, the end of largest diameter being in thecommon carotid artery and the end of smallest diameter being in theinternal carotid artery.
 3. The method of claim 1 wherein the stent isformed of a shape memory material so as to be self-expanding, the stentstructure having an unexpanded configuration and an expanded memorizedconfiguration, the unexpanded configuration having an average diameterless than that of the expanded configuration, the expanded configurationhaving a diameter which varies along the length of the stent from afirst end region, to a middle region, and to a second end region,wherein the second configuration diameter is smaller in said first endregion, larger in said middle region than in said first region, andlargest in said second end region than in said middle region or saidfirst end region.
 4. The method of claim 3 wherein the stent isimplanted in the internal carotid with the end of largest diameter atthe ostium thereof and the end of smallest diameter in healthy tissuedownstream of the ostium.